The present invention relates to a compound superconducting wire suitable for use in superconducting magnets of high magnetic field and a method for manufacturing the same. Particularly, the present invention is advantageously applicable to an Nb-Sn compound superconducting wire and a manufacturing method therefor.
The term "precursor of superconducting wire" or "superconducting wire precursor" as used herein means a wire prior to undergoing a heat treatment, i.e., a wire prior to being imparted with a superconductivity. The heat treated precursor to impart a superconductivity, namely the wire converted into superconductor by the heat treatment, is termed a "superconducting wire".
Hitherto, superconducting wires have been manufactured by a method so-called "internal diffusion method".
An Nb.sub.3 Sn superconducting wire prepared by the internal tin diffusion method is known, for instance, from Japanese Patent Publication Kokoku No. 16141/1986. FIGS. 23 and 24 are explanatory sectional views respectively illustrating an Nb.sub.3 Sn superconducting wire precursor prior to undergoing a heat treatment according to a conventional internal diffusion method described in Japanese Patent Publication Kokoku No. 16141/1986 and an Nb.sub.3 Sn superconducting wire after undergoing the heat treatment. In FIG. 23, denoted at numeral 41 is the superconducting wire precursor prior to the heat treatment, at numeral 43 Nb base metal filaments to be converted into a superconductor by the heat treatment, at numeral 44 barrier layer such as made of Ta, at numeral 45 stabilizing layer such as made of oxygen free copper, at numeral 46 Cu base metal material, and at numeral 47 Sn base metal material. In FIG. 24, denoted at numeral 48 is the superconducting wire after the heat treatment, at numeral 49 Nb.sub.3 Sn filaments having a superconductivity, and at numeral 50 low-Sn-concentration bronze.
The superconducting wire 48 is obtained by subjecting the superconducting wire precursor 41 to a heat treatment at a high temperature (typically ranging from 600.degree. to 800.degree. C.) to produce an Nb.sub.3 Sn compound in the Nb filaments 43.
The conventional method for manufacturing an Nb.sub.3 Sn superconducting wire using the internal diffusion method is as follows. First, an Nb base metal material is inserted into a Cu tube and processed to decrease the area in section to a certain diameter thereby giving a single core wire. This single core wire is cut into pieces having an appropriate length and a plurality of these wire pieces are stuffed into a container made of Cu. In the center portion of this container is disposed a Cu base metal material such as a Cu rod or a bundle of Cu wires. Air in the container is evacuated, a cover is welded to the container to seal it up, and the thus treated container is extruded. Thereafter the Cu base metal material in the center portion of the container is mechanically formed with an aperture. An Sn base metal material is inserted into this aperture, and the Cu container is circumferentially covered with a tube made of Ta or Nb, which is further covered with a Cu tube. The resultant is processed to reduce the cross-sectional area, typically is drawn to a small size. When it is desired to produce a superconducting wire having a heavy current capacity, a plurality of the thus obtained composite wires may be inserted into a Cu tube and then drawn to reduce the section area. After drawing the wire to a final diameter, it is twisted and subjected to a heat treatment. This heat treatment causes Sn to diffuse into Cu existing therearound to form Cu-Sn alloy and further to react with the Nb base metal filament to produce Nb.sub.3 Sn either partially or entirely.
The superconducting wire precursor in the aforesaid internal diffusion method has a structure in which the Nb base metal filaments and a core of the Sn base metal material are embedded in the Cu base metal material. In particular, in order to increase as large as possible the critical current density (Jc) which is one of the characteristics of superconductivity, the Nb base metal filaments are embedded in the Cu base metal material as tightly as possible. The superconducting wire when cooled to the temperature of liquid helium is capable of allowing heavy current to flow therein without producing any electrical resistance.
As described above, in the compound superconducting wire manufactured by the prior art internal diffusion method the Sn base metal material is disposed at the center of the module and, hence, the space between adjacent Nb.sub.3 Sn filaments is as narrow as about a half of the spacing between such filaments arranged in accordance with a usual bronze method. For this reason the Nb base metal filaments tend to come into contact with each other to combine to each other when the superconducting wire precursor is heat-treated, thus resulting in increase of the effective filament diameter (d.sub.eff), which greatly influences the electrical characteristics of the superconducting wire. The effective filament diameter is a value given by d.sub.eff =3.pi..DELTA.M/4 .mu..sub.o Jc where the sample is in the columnar form, .DELTA.M represents the width of magnetization of the superconducting wire, and Jc represents the critical current density in these conditions. As a result, a problem arises that although the resulting superconducting wire suffers no problem with respect to DC current, it suffers a large hysteresis loss when pulse current is made to flow therein with the result that the superconducting coil generates heat to degrade the stability thereof.
Further, since the Sn base metal material is centrally disposed in the internal diffusion method, preheating for Sn diffusion produces a gradient in Sn concentration. Accordingly, the composition of the Nb.sub.3 Sn filements varies depending on the Sn concentration. This poses another problem that the n value, which is one of the characteristics of superconductivity, is undesirably decreased. The n value is an index representing the longitudinal homogeneity of a superconducting wire and appears in the formula, V.varies.I.sup.n. The superconducting characteristics of the wire become more excellent with increasing n value.
It is, therefore, a primary object of the present invention to provide a compound superconducting wire wherein the effective filament diameter thereof is decreased to a large extent with minimizing the decrease in the critical current density Jc thereof, and the n value is increased.
A further object of the present invention is to provide a process for producing a superconducting wire, especially an Nb-Sn compound superconducting wire, having a high critical current density Jc, an improved effective filament diameter and an improved uniformity in composition of superconductive compounds in the longitudinal direction of wire.
A still further object of the present invention is to provide an improved internal diffusion process for producing a superconducting wire which enables to prevent filaments of a metal to be converted into a superconductive compound from contacting each other during the heat treatment without decreasing the number of the filaments arranged in a matrix metal.
Another object of the present invention is to provide a precursor of compound superconducting wire which can provide a superconducting wire having an improved critical current density Jc, a decreased hysteresis loss and an improved uniformity in composition of superconductive compound by a heat treatment in a shortened period of time.
These and other objects of the present invention will become apparent from the description hereinafter.